Myomere

Abstract: Sonomicrometrics of in vivo axial strain of muscle has shown that the swimming fish body bends like a homogenous, continuous beam in all species except tuna. This simple beam-like behavior is surprising because the underlying tendon structure, muscle structure and behavior are complex. Given this incongruence, our goal was to understand the mechanical role of various myoseptal tendons. We modeled a pumpkinseed sunfish, Lepomis gibbosus, using experimentally-derived physical and mechanical attributes, swimming from rest with steady muscle activity. Axially oriented muscle-tendons, transverse and axial myoseptal tendons, as suggested by current morphological knowledge, interacted to replicate the force and moment distribution. Dynamic stiffness and damping associated with muscle activation, realistic muscle force generation, and force distribution following tendon geometry were incorporated. The vertebral column consisted of 11 rigid vertebrae connected by joints that restricted bending to the lateral plane and endowed the body with its passive viscoelasticity. In reaction to the acceleration of the body in an inviscid fluid and its internal transmission of moment via the vertebral column, the model predicted the kinematic response. Varying only tendon geometry and stiffness, four different simulations were run. Simulations with only intrasegmental tendons produced unstable axial and lateral tail forces and body motions. Only the simulation that included both intra- and intersegmental tendons, muscle-enhanced segment stiffness, and a stiffened caudal joint produced stable and large lateral and axial forces at the tail. Thus this model predicts that axial tendons function within a myomere to (1) convert axial force to moment (moment transduction), (2) transmit axial forces between adjacent myosepta (segment coupling), and, intersegmentally, to (3) distribute axial forces (force entrainment), and (4) stiffen joints in bending (flexural stiffening). The fact that all four functions are needed to produce the most realistic swimming motions suggests that axial tendons are essential to the simple beam-like behavior of fish.

Abstract: The anatomy of the Upper Devonian jawless vertebrate Euphanerops longaevus Woodward, 1900, from the Escuminac Formation of Miguasha, Quebec, Canada, is described on the basis of new specimens, some of which display what is regarded here as an extensively mineralized endoskeleton that is essentially made of calcium phosphate, with traces ofdiagenetic calcite and silicate. Most of the mineralized elements of E. longaevus display the same spongiose microstructure, which notably occurs in such undoubtedly endoskeletal elements, as the fin radials, and this suggests that they all are actually endoskeletal elements. Their structure consists of groups of large, generally paired, ovoid spaces that recall the chondrocytes of lamprey cartilage, and are therefore referred to as "chondrocyte spaces". The latter are surrounded by a shell of "mineralized territorial matrix", and cemented by a finely spherulitic "mineralized interterritorial matrix" that extends between them. The question whether this mineralized tissue is an unusual form of biogenic calcified cartilage, or an authigenic, microbially induced post-mortem phosphatization, is discussed, but no definite answer is proposed. The snout of E. longaevus displays three "head stains" that may be either the imprints of the collapsed median olfactory organ and paired eyes, or traces of cartilaginous plates arming the snout. In large specimens, these are followed posteriorly by a large patch of mineralized tissue tentatively interpreted as a braincase. The branchial apparatus consists of an elongated, cone-shaped "basket", composed of at least 30 vertical, sinuous gill arches, and extending from beneath the presumed braincase to the anal region. The gill arches bear a large number of more or less horizontal gill ray-like mineralized rods, which probably supported the gill filaments. The gill arches seem to have been attached dorsally and ventrally to series of massive endoskeletal elements, referred to as the "copular elements". The ventral series of copular elements is prolonged anteriorly by a median "anterior ventral rod", which extends to a ring-shaped structure, the "annular cartilage". The homology of the latter to the annular cartilage of lampreys remains uncertain. The position of the heart remains problematical, despite the possible presence of a pericardiac cartilage at the rear of the branchial basket. The viscera were housed dorsal to the branchial apparatus, and comprised a large stomach containing fine-grained sediment, but the organization of the digestive tract and its relations to the branchial apparatus remains unknown in detail. The axial skeleton clearly displays complete dorsal and ventral series of arcualia, ventrally to which extends a series of elements referred to as "haemal series". The anal fin radials are supported by large "anal fin supports". E. longaevus is regarded here as having possessed thin paired fin radials, arranged in ventrolateral series, which diverge anteriorly, like in anaspids, but this species is unique among vertebrates in having paired fins that extend ventrolaterally to the branchial apparatus. Many anatomical features of E. longaevus remain nevertheless unexplained, such as the "white line" and the "black lines", tentatively interpreted here as blood vessels. Peculiar mineralized elements, referred to as the "intermuscular elements" and "diffuse mineralized matter", have no equivalent in other vertebrates and may either have been endoskeletal elements housed in intermuscular connective tissues of the trunk, or haphazardly distributed authigenically phosphatised soft tissues. The oblique, elongated imprints, variously referred to as "scales", or "myomere imprints" in previous descriptions, are only seen in the smaller and poorly mineralized individuals but their nature remains unknown. The sediment in the stomach contents rather suggests microphagous bottom feeding.

Abstract: Morphological characteristics are known for 62 species of cyprinid larvae in 22 genera occupying waters of North America east of the Continental Divide. Currently known diagnostic characters are presented for most of these species. Species are arranged into groups according to their relative preanal length, eye shape, preanal myomere number, and ventral pigmentation. Descriptive literature is noted, as are errors in the literature. Practical considerations for making identifications, species in need of description, and characters to be investigated are discussed.

Abstract: Growth of laterarl muscle in the teleost fish Sparus aurata (L.) was examined from hatching to juvenile by a basic morphofunctional approach that takes into account structural and ecophysiological aspects and combines in vivo observations and LM and TEM microscopic analysis. As shown in most teleost fishes, muscle growth proceeds by a double mechanism of hyperplasia and hypertrophy that contribute differentially to the overall development of the lateral muscle, giving rise in each myomere to a typical pattern of structurally and functionally different fibre types (slow-red and fast-white fibres, plus pink intermediate fibres) in a nerve-dependent process. During larval life the muscle growth takes place mainly due to hyperplastic growth at the level of specific proliferative zones of the myomeres, from which slow, pink and white muscle fibres are derived. In those species that reach a large adult size a new typical hyperplastic process disseminated throughout the fast white muscle layer takes place during post-larval life. In contrast, hypertrophic growth occurs in all stages, but is the dominant mechanism of muscle growth only in juvenile and adult. The suitable recruitment of the different fibre types enables the fish to optimize its performances according to specific functional and metabolic requirements related to the swimming behaviour and hydrodynamic regimes. The different mechanisms of growth are here analysed in their detailed structural and ultrastructural aspects in order to interpret their adaptive significance in the light of the fish life cycle, with particular reference to locomotion and feeding behaviour.